Term usage: "Blow off valves" go by several names, among them are compressor bypass valve (CBV), air by-pass valve, bypass valve (BPV), blow off valve (BOV), Diverter valve, and possibly a few others. BOV is the common and incorrect term that lumps true blow off valves and bypass valves under the same term. For the sake of correctness, this post will refer to either aftermarket BOV, aftermarket BPV or OEM BPV as these are the most correct terms.

What is the function of a blow off valve (BOV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to releases the air to the atmosphere.

What is the function of a bypass valve (BPV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to recirculate the air back into the intake before the turbo inlet, but after the airflow sensor.

What is the purpose of a BOV/BPV? When the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and in the absence of a BOV/BPV, a pressure wave will travel back to the turbocharger. The result is that the compressor wheel will stall (a phenomenon known as “compressor surge”) and slow down very quickly. This is hard on the bearings and decreases the turbo’s lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened.

Are aftermarket BOVs necessary with Subaru turbos? No. The OEM BPV is perfectly fine up to 20psi of boost. For applications using higher boost levels, an aftermarket BOV/BPV should be considered.

Can I mod my stock BPV to hold higher boost? Yes. This link gives detailed instructions on how to do so.

Is the STi BPV better than the WRX BPV? No. They are the same. However, the JDM STi BPV will hold more boost as it is physically different than both the USDM STi BPV and the WRX BPV. The specific PSI rating of the JDM STi BPV is unknown, but users have reported it is good up to 25 PSI.

Is an aftermarket BPV better than the stock BPV? No. Unless you are considering an aftermarket BPV solely for the purposes of holding higher boost levels. An aftermarket unit should sound just like the OEM unit.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" aftermarket BOVs. Obviously, there may have been bad ones sold, but not enough to report as "bad" overall.

What are the different types of aftermarket BOVs/BPVs? Different manufacturers use different methods. There are three basic types:
1. Aftermarket BPV: Similar in function to the OEM BPV where 100% of the air is recirculated.
2. Atmospheric BOV: 100% of the air is vented to the atmosphere.
3. Hybrid BOV: These depend on the manufacturer and end user settings. These can either be adjustable or manufacturer set for different percentages of atmospheric/recirculation dumping. They can also be set to work as recirculation during lower boost conditions and 100% atmospheric during higher boost conditions.

Are there any downsides to aftermarket BOVs? There have not been significant amounts of problems with BOVs. Aftermarket BOVs can and do require some light end user maintenance to keep them performing perfectly. For aftermarket hybrid BOVs that have end user defined settings, there will be an initial period of adjustment to obtain the desired recirculation/atmospheric ratio. As well, most aftermarket BOVs will require "tuning" (usually via supplied washers, a screw, or other mechanism on the BOV itself) to allow them to idle correctly and blow off at the right time.

Are there any negative effects with aftermarket BOVs? Yes. The downside of releasing the air to atmosphere is that it has already been metered by the mass air sensor, and when it blows off, the ECU will be injecting the wrong amount of fuel into the cylinders. The engine temporarily runs rich, meaning too much fuel is injected into the cylinders. On most tunes the target A/F under boost is @11.1:1 or so. Say you are at 11.1:1, then you shift and it vents. It will swing rich, typically to around 9.5:1. That is not that rich and this period lasts for under one second...again, nothing to write home about.

This temporary rich condition isn’t usually that harmful. Technically, it can eventually foul spark plugs and even clog the catalytic converter as unburned fuel on the catalytic converter burns very hot, and too much of it can melt the cat. The odds of either of these two conditions actually happening is very, very low though, but that's the theory.

As to blanket "you'll run rich" statements, a BOV will only run "rich" during hard acceleration and shifting as 99% of the time it stays closed.

Can my tuner or engine management tune out this rich period? Yes. There are some forms of engine management that can tune this out. Buying your engine management solely for this purpose is a poor method of choosing an engine management system though.

What about a 50/50 or BOVs that you can portion the VTA portion? This is a bad analogy, but if a BOV is a person in a wheelchair, a 50/50 BOV is a person in leg braces. It's not as bad, but not good enough to say bolt it up. If you find a deal on one or happen to like the sound of a particular model, go for it, but don't think you are doing your car better vs. a 100% VTA model.

Won't I be labeled as a ricer? It's your car, do with it what you like, but be aware that many people are prejudiced against BOVs just like neon underbody lights and other accessories. Be aware that a BOV can be seen as a dinner bell for street racers though. It attracts attention both good and bad.

So a BOV is not bad then right? Not so fast...most BOVs leak. Even ones that are religiously maintained, installed, and are from quality manufacturers. A leak in essence means less boost and less power or your car will be working harder to produce boost than it means, which can decrease reliability.

How do I set up my BOV to not leak/stand less chance of leaking? Read the BOV manual/instructions first off. Learn how to adjust it. If it doesn't have a manual, just look at the valve. You can almost always figure out how to adjust it. Some have screws, some use washers under the spring, others use replacement springs. Now that you know how to adjust it, here is what you do:

First off, start the car, get out and open the hood. If you can see the piston of the valve (look in the hole it vents out of if its a VTA valve), check to see if it is moving at all at idle, or worse, partially open. You want that piston rock solid, not moving at idle.
Now give it a little gas. It shouldn't move when you step on it, and when you lift, it should pull up a bit. Depending how hard you hit the gas, it will either JUST move, or fully open.
If its doing anything but what it should be, adjust it a little tighter.

Now go drive the car

When you shift at light throttle, is it venting? If so, is it a pure straight vent, or do you get a little bit of a "chufchchchc" noise right before it vents? That noise is compressor surge. Which isn't as bad as some people like to think and say. Ideally, you are looking to hear a little surge then a vent. Or no sound at all. Either is ok. Now drive it hard, does it surge a little or just straight vent?
Get out of the car and tighten the valve and drive it again. Repeat this until you get a LOT of surge when you lift under boost. Now start loosening it until you JUST start to hear no almost surge.
What the surge means is that it fought opening just a little bit. If you hear a LITTLE surge just as it begins to vent, that means that the valve is holding itself shut as tightly as it is capable of, without being so tight that it doesn't vent properly.

I have had XXXX brand valve for years, it has never leaked. How do you know? By looking at your boost gauge? Looking at the boost levels in your datalog? Neither of those prove that the valve isn't leaking. The advice above tells you how to set it and KNOW for 100% sure.

But compressor surge will kill my turbo! Unless its really bad, it really isn't going to kill it.

I have had XXXX brand valve for years, it has never leaked. How do you know? By looking at your boost gauge? Looking at the boost levels in your datalog? Neither of those prove that the valve isn't leaking.

Where do I buy aftermarket BOVs/BPVs? Every Subaru/Import performance store sells BOVs. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install aftermarket BOVs/BPVs? Allow around 1 hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

How do I install BOVs/BPVs? Refer to the BOV/BPV manufacturer's instructions. For BOVs/BPVs without instructions, below is a link to one of the better known installation instructions:
scoobymods.com instructions (with photos)

If you are wanting to be 100% sure you aren't losing any power from a BOV, and you are running under about 22PSI, you are best off sticking with a stock unit.